1. Field of the Invention
This invention pertains to wire stripping apparatus with V type cutting blades.
2. Description of the Prior Art
Industry utilizes two basic types of cutting blades for high production stripping of insulation from insulated electrical conductors, the die type and the V type. FIG. 1a illustrates a pair of typical die type blades 2 and 4. In the die type design, the knife blades 2 and 4 form two halves of a counterbored hole 6 and a smaller through hole 8. See FIG. 1b. The two blades are closed over the insulation of a specifically sized insulated conductor at the desired location thereon. The die type design possesses the advantages of closely conforming to the configuration of the conductor around the full periphery thereof and of having a depth of cut controlled by the contacting of leading edges 12 to prevent nicking and scoring the conductor. Nicks are undesirable because they decrease electrical current carrying capacity, and they produce stress concentrations which decrease the tensile strength of the conductor. However, die type blades suffer from four disadvantages. The first is that a pair of die blades is designed to be used with only one size of insulated conductor. Secondly, some insulation is usually squeezed between the conductor and the periphery of hole 8, which may prevent the blades from fully cutting through the insulation. As a result, the uncut insulation must be torn from the parent insulation. The physical characteristics of some insulation materials make them difficult to tear, so it is desireable that the blades cut completely through the insulation. The third disadvantage is that die type blades have only limited ability to gather off-center insulated conductors to the cutting edges of the blades. In many applications, special wire guides in the form of mechanical fingers are required to ensure that the insulated conductor is on the center line of the closed blades. The fourth disadvantage is that die type blades are difficult and expensive to manufacture. Referring to FIG. 1a it is apparent that the intricate shapes of the blades 2 and 4 are costly to produce.
FIGS. 1c and 1d show a conventional V type blade 1 which is in widespread commercial use. The blade includes a V-shaped opening 3 which terminates in a generally round cutting edge 5. Blades are used in pairs to close over an insulated conductor (not shown) for gathering and directing it to the cooperating cutting edges. Ideally, the blade cutting edges slice the insulation from the insulated conductor while avoiding contact with the conductor.
It is desirable that the cutting edge 5 be circular in the plane coinciding with blade face 10. That is because insulated conductors are fed to the blades in a direction perpendicular to the face 10, and the periphery of a conductor is a circle. Any cutting edge contour other than a circle does not conform to the conductor circumference and thus does not optimally cut the insulation. If the non-circular cutting edge closes over the conductor so as to avoid contact with the conductor, some insulation remains uncut. The uncut insulation must be torn away during the stripping process. If the non-circular cutting edge is closed over the conductor so that substantially all the insulation is cut, a portion of the cutting edge will contact and nick the conductor.
A common method of manufacturing conventional V type blades is illustrated in FIGS. 2a and 2b. Several blade blanks 7 are secured in a blade fixture 9 such that faces 10 of the blanks make an angle A with the fixture. A blade manufacturing tool, such as an abrasive wheel 11, is fed through the blanks 7 in the direction of arrow 13. The periphery of the abrasive wheel 11 is dressed to a circular cross-section, as indicated by reference letter R. The resulting blade is as shown in FIGS. 1c and 1d. It will be understood that the aforedescribed method may be used to manufacture compound angled stripping blades as disclosed in my co-pending U.S. patent application Serial No. 651,830.
The basic shortcoming of conventional blades 1 is that the cutting edge 5 is not circular in the plane coinciding with face 10. Rather, the cutting edge is elliptical in that plane. That may be demonstrated with the aid of FIG. 3. In FIG. 3, the plane a--a represents a blade blank 7 held in a fixture so that the faces 10 are at an angle A to the direction of motion 17 of wheel 11. The semi-circle 15 represents the transverse contour of the wheel periphery and also the transverse contour of the path formed in the blank as the wheel passes through the blank. The travel of the semi-circular periphery of the grinding wheel in the direction of arrow 17 produces half of a right circular cylinder 19 having longitudinal axis 20. The intersection of the half cylinder 19 with the plane a--a results in half of an ellipse 21. Thus, conventional grinding techniques produce cutting and stripping blades having elliptical cutting edges corresponding with ellipse 21 in the plane of face 10 rather than circular cutting edges.
It will be understood that other manufacturing methods are also in more or less widespread use. Fabricating stripping blades by means of wheel dressing blocks, crush rolls, electrodischarge machining, and pantograph and/or optical grinding are well known in the art. In the various manufacturing techniques the manufacturing tool is fed into the blank or the blank is fed into the tool so as to remove material in the form of a partial circular cylinder having a longitudinal axis which is at an angle to the blade face 10. Standard techniques more easily make simple geometric forms such as circles. Consequently, a circular grinding wheel cross-section is used which produces an elliptical cutting edge with respect to the axis of the insulated conductor to be cut and stripped. For certain conductor diameters and angles, the blade cutting edge 5 is only slightly elliptical. For that reason, elliptical cutting edges have been satisfactory for some applications. Nevertheless, a need exists for an insulation cutting and stripping blade having a cutting edge which is circular with respect to the contour of the conductor and for a tool and method for making the cutting blade.